Coating composition for steel sheets having zinc and zinc alloy coating layer, method for forming coating layer using the coating composition and steel sheet having the coating layer formed thereof

Abstract

A coating composition for zinc- and zinc alloy-coated steel sheets includes: 1 to 30% by weight of hypophosphorous acid, 0.1 to 10% by weight of manganese, 0.01 to 5% by weight of a polyvinyl compound, zinc oxide and the balance of water, and further comprises 20% by weight or less of alcohol, when necessary, wherein the zinc oxide is included in such a content that, when 50 Ml (milliliters) of the coating composition is neutralized and titrated with 0.1N NaOH, the NaOH used for the neutralization titration is present in an amount of 10.0 Ml (milliliters) or less, and a content of free acid in the coating composition is adjusted to such an amount that, when 50 Ml (milliliters) of the coating composition is neutralized and titrated with 0.1N NaOH, the NaOH used for the neutralization titration is present in an amount of 10.0 Ml (milliliters) or less. The coating film formed on the steel sheet using the coating composition shows excellent properties such as lubricating property, adhesion and coating removal property, and also has a fine surface appearance.

1. A zinc- and zinc alloy-coated steel sheet having a coating film formed with a coating composition for zinc- and zinc alloy-coated steel sheets comprising: 1 to 30% by weight of hypophosphorous acid, 0.1 to 10% by weight of manganese, 0.01 to 5% by weight of a polyvinyl compound, zinc oxide and the balance of water, wherein the zinc oxide is comprised in such a content that, when 50 ml (milliliters) of the coating composition is neutralized and titrated with 0.1N NaOH, the NaOH used for the neutralization titration is present in an amount of 10.0 ml (milliliters) or less, and a content of free acid in the coating composition is adjusted to such an amount that, when 50 ml (milliliters) of the coating composition is neutralized and titrated with 0.1N NaOH, the NaOH used for the neutralization titration is present in an amount of 10.0 ml (milliliters) or less,
 wherein a dried coating weight of the coating composition coated onto the coating layer is in a range of 100 to 2000 mg (milligrams)/m 2  (square meters) per one side of the steel sheet. 
 
     
     
       2. The zinc- and zinc alloy-coated steel sheet of  claim 1 , wherein a dried coating weight of the coating composition coated onto the coating layer is in a range of 200 to 400 mg (milligrams)/m 2  (square meters) per one side of the steel sheet. 
     
     
       3. The zinc- and zinc alloy-coated steel sheet of  claim 1 , wherein the coating composition further comprises 20% by weight or less of alcohol, based on the total weight of the coating composition. 
     
     
       4. The zinc- and zinc alloy-coated steel sheet of  claim 3 , wherein the content of the alcohol is in a range of 3.0 to 20% by weight. 
     
     
       5. The zinc- and zinc alloy-coated steel sheet of  claim 3 , wherein the content of the alcohol is in a range of 5.0 to 10% by weight.